The invention relates to a glass which has a good transparency to UV radiation. The glass is also intended to have a good hydrolytic resistance and a low thermal expansion, it being intended that the thermal expansion is such that the glass can produce a pressurized glazing with Al.sub.2 O.sub.3 and, in addition, can form a seal with tungsten.
In order to ensure an optimum economic utilization of the glass, the properties of the glass should be such that it can be used, on the one hand, as window glass of EP-ROMs and, on the other hand, as a lamp for UV irradiation.
The term EP-ROM is the abbreviation for Erasable Programmable - Read Only Memory. This is a specific "read-only semiconductor storage device". In this semiconductor, the stored program can be erased by ultraviolet (UV) radiation. To hold the semiconductor, use is made of a housing of aluminum oxide ceramic which contains a window of highly UV-transparent glass. The UV transparency of the glass should be at least 70% for the wavelength 253.7 nm and a glass thickness of 1 mm. The glass window has to be sealed into the Al.sub.2 O.sub.3 housing so that the interior of the housing is isolated in a gastight manner. This is achieved by using a glass with a lower thermal expansion than the Al.sub.2 O.sub.3 ceramic. As the glass-ceramic seal cools, the ceramic housing shrinks onto the glass windows, seals the interior space in a gastight manner and places the glass under compressive stress. In order to guarantee the long-term use of the EP-ROMs even in countries with high atmospheric humidity, the glass windows must have a good hydrolytic resistance.
UV-transparent lamps are needed, for example, for therapeutic treatment or for sterilization. UV rays below 254 nm are harmful for therapeutic purposes, while UV rays up to 185 nm are important for sterilization. The production of lamps assumes that the glass used can form a seal with tungsten, Vacon 10.RTM. (an Ni/Fe/Co alloy) or molybdenum.
A UV-transparent glass is known from British Patent Specification 589,533. The glass described there contains 50-70% by weight of SiO.sub.2, 4-10% by weight of Al.sub.2 O.sub.3, 1-5% by weight of alkali-metal oxide and 20-40% by weight of B.sub.2 O.sub.3. The teaching therein states that the alkali-metal oxide content should be kept as low as possible to produce a UV-transparent glass.
A further UV-transparent glass is known from the Japanese Patent Application 85-21830. This has the following composition: 60-70% by weight of SiO.sub.2, 4-8% by weight of Al.sub.2 O.sub.3, 18-25% by weight of B.sub.2 O.sub.3, 6-11% by weight of Li.sub.2 O+Na.sub.2 O+K.sub.2 O, 0-4% by weight of MgO+CaO+SrO+BaO+ZnO and 0-3% by weight of F.sub.2. The glass has a coefficient of thermal expansion .alpha. of 5.0 to 5.8.times.10.sup.-6 K.sup.-1 at 30.degree. to 380.degree. C.
The glasses known from the literature can in each case be used for specific purposes, such as, for example, for producing UV lamps or EP-ROM windows. However, no UV-transparent glass is known which is suitable for various possible applications.